FIG. 10 is a horizontal sectional view of a conventional in-line type color picture tube having a bipotential type electron gun. In this drawing, a fluorescent screen 2 is formed inside a faceplate 1 by applying fluorescent materials, and a shadow-mask 3 which allocates electron beams to the fluorescent screen 2 is installed at a predetermined distance from the fluorescent screen 2.
In the neck portion of the picture tube are installed three cathodes 4, 5, 6, a first electrode 10, a second electrode 11, a third electrode 12 and a fourth electrode 16 which form the main electron lenses, and a shielding cup 20, in serial arrangement. Each of the first electrode 10, the second electrode 11 and the third electrode 12 is provided with apertures corresponding to the cathodes 4, 5, 6, and the axes of these apertures coincide with that of the cathodes 4, 5, 6.
Inner cylinders 13, 14, 15 are installed on the right side of the third electrode 12 and inner cylinders 17, 18, 19 are installed on the left side of the fourth electrode 16. Among them, the axes of the inner cylinder 13, 14, 15 of the third electrode 12 coincide with the axes 7, 8, 9 of the cathodes respectively and the axis of a central inner cylinder 18 of the fourth electrode 16 also coincides with the axis 8 of the central cathode 5. However, the axes of the outer and inner cylinders 17, 19 of the fourth electrode are slightly deviated from the axes 7, 9 of the outer cathodes toward the outside, respectively.
The electron beams injected from the cathodes 4, 5, 6 proceed to the main electron lenses along the axes 7, 8, 9. Here, the electric potential of the third electrode 12 is lower than that of the fourth electrode 16, and the electric potential of the shielding cup 20 is the same as that of the fourth electrode 16.
The central apertures of the third and the fourth electrodes are coaxial with the central inner cylinders 14, 18. Because these inner cylinders prevent the influences arising from the nonsymmetry of the periphery of the electrode, the central main electron lens is of symmetrical shape. Accordingly, the central electron beam, i.e., the green beam is concentrated by the symmetrical main electron lens and proceeds straight along the axis 8. On the other hand, the outer main electron lenses are of nonsymmetrical shape because of the deviation of the axes of the outer and inner cylinders 17, 19 of the fourth electrode 16 from the axes of the outer inner cylinders 13, 15 of the third electrode 12. By this nonsymmetry each of the outer electron beams, i.e., the red beam, and the blue beam pass through the portions deviated from the axes of the center of the lenses at the divergence lens region formed by the fourth electrodes 16. These two beams are not only concentrated but are also deflected toward the central beam by the nonsymmetric main electron lenses thereby performing static converging of the three electron beams.
Thus, the converged electron beams reach the fluorescent screen 2 through the shadow-mask 3.
Among the factors that affect the focusing characteristic of the picture tube, are the magnification and aberration of the main electron lenses. These two factors are influenced by the concentration intensity of the lenses.
If focusing distance of the electron beam is kept constant, the magnification of the lens is lowered as the concentration intensity of the lens is weakened, and the angle of incidence declines as the spread of the beam inside the lens is confined to some extent in order to suppress enlargement of the deflection aberration.
If the concentration intensity of the lens is weakened, while the magnification of the lens and the spherical abberation is lowered, the focusing characteristic is improved. One of methods of weakening the concentration intensity is to enlarge the diameter of the inner cylinders corresponding to the apertures of the third electrode and the fourth electrode forming the main lens.
Generally, provided that the electric current or the luminance remains on the same level, the spherical aberration may be reduced in order to make the beam spot smaller. It can be shown from the equation: ##EQU1## wherein D.sub.T represents the diameter of the beam spot on the fluorescent screen, D.sub.X represents the diameter of the beam spot which is determined by the magnification of the lens, D.sub.SA represents the spread of the beam spot diameter because of the spherical aberration, and D.sub.SC represents the spread of the beam spot diameter by the mutual repulsion of the space charge. From this equation it can be seen that the spread D.sub.SA of the beam spot diameter, because of the spherical aberration, affects the diameter D.sub.T of the beam spot on the fluorescent screen.
The effective diameters of the main electron lenses should be enlarged if the spherical aberration is to be reduced. However, there is an ultimate restriction on enlarging the diameter of the apertures on the third and the fourth electrodes in order to enlarge the effective diameters of the main electron lenses. As shown in FIG. 1, the main electron lenses of an in-line type electron gun each corresponding to the red, green and blue beams are all arranged in a line on the same plane. Thus, the diameter of the aperture of the electrode should be smaller than one third of the inside diameter of the neck portion which surrounds the electron gun.
A method for enlarging the diameter of apertures of the electrode is disclosed in Japanese Patent Laid-Open No. Sho-55-17963. According to this method, the diameters of apertures are set larger than the eccentric distance between the neighboring apertures, the overlapped portions by the apertures are communicated with each other, and partitioning plates are interposed between the apertures in order to correct the electric potential.
But this method also has a problem such that the diameter of the aperture L is limited by the equation: EQU L=H-2S
wherein H represents the horizontal (in the direction on which the apertures are arranged) length of the third electrode and S is represents as the eccentric distance between the neighboring apertures. In practice, problems arising from manufacturing the electrode render the value of the diameter of as described the aperture L smaller than the above.
An electrode structure, as shown in FIG. 11, proposed to accomplish the same effect as that obtained from the enlarging of the diameters of the aperture on the electrode is disclosed in another Japanese Patent Laid-Open No. Sho-58-103752.
This electrode structure, shown in FIG. 11, is provided with electrode plates 112, 122 inside the third electrode G3 and the fourth electrode G4, respectively, and recessed to the extent of d.sub.1, and d.sub.2 from their faces. Apertures 113, 113', 114, 123, 123', 124, formed on the electrode plates 112, 122, are of elliptical form with their major axes a.sub.1, a.sub.2 and their minor axes b.sub.1, b.sub.2. Inner cylinders such as those shown in FIG. 10 are not adapted in this structure.
By means of this electrode structure, a higher electric potential of the G4 electrode permeates G3 electrode and a lower electric potential of the G3 electrode permeates the G4 electrode, which results in the same effect as that obtained from enlarging the diameters of the apertures of the electrodes. Namely, this effect is equal to that obtained from enlarging effective diameters. Here, the apertures are of elliptical form to remove the astigmatism arising from the permeation of the electric potential that is stronger in the perpendicular direction than in the horizontal direction.
However, the problem of this electrode structure lies in manufacturing. The forming of the electrode plates 112, 122 within the outer electrodes 111 and 121 integrally respectively, as shown in FIG. 11, is not readily performed by a simple process such as a pressing process. Instead, it must undergo a complicated manufacturing process such as sintering the powdered electrode materials. The focusing characteristic is seriously influenced by the accuracy in the form of the apertures and the location of the electrodes, as will be explained later. Accordingly, the aforesaid manufacturing process must be followed by additional processes in order to ascertain the accuracy in the form of the apertures and the location of the electrodes as required. These complicated manufacturing processes act as a cost increasing factor, so that this electrode structure is not applicable to mass production.
In order to solve the above problem, in practice the electrode structure of FIG. 11 had to be reformed into the structure as shown in FIG. 12. The electrode structure of FIG. 12 can be formed by assembling the outer electrode 111' and the electrode 112', each separately fabricated by pressing processes. This reformed electrode structure is supposed to result in the same effect as that of the original electrode structure. However, even by using a jig, etc., for assembling the outer electrode 111' and the electrode 112', it is not easy to maintain the uniformity of the distance Df and to align the axes of the apertures with those of the cathodes. These difficulties again result in the problems of accuracy in the form of the apertures and the location of the electrodes as required.